You can do that for a PoC, but that's a bad idea for any production workload. You'd want at least three nodes with OSDs to use the default RF=3 replication. You can do RF=2, but at the peril of your mortal data.
I'm not sure I agree - I think size=2, min_size=2 is no worse than RAID1 for data security.
size=2, min_size=2 *is* RAID1. Except that you become unavailable if a single drive is unavailable.
That isn't even the main risk as I understand it. Of course a double failure is going to be a problem with size=2, or traditional RAID1, and I think anybody choosing this configuration accepts this risk.
We see people often enough who don’t know that. I’ve seen double failures. ymmv.
As I understand it, the reason min_size=1 is a trap has nothing to do with double failures per se.
It’s one of the concerns.
The issue is that Ceph OSDs are somewhat prone to flapping during recovery (OOM, etc). So even if the disk is fine, an OSD can go down for a short time. If you have size=2, min=1 configured, then when this happens the PG will become degraded and will continue operating on the other OSD, and the flapping OSD becomes stale. Then when it comes back up it recovers. The problem is that if the other OSD has a permanent failure (disk crash/etc) while the first OSD is flapping, now you have no good OSDs, because when the flapping OSD comes back up it is stale, and its PGs have no peer.
Indeed, arguably that’s an overlapping failure. I’ve seen this too, and have a pg query to demonstrate it.
I suspect there are ways to re-activate it, though this will result in potential data inconsistency since writes were allowed to the cluster and will then get rolled back.
Yep.
With only two OSDs I'm guessing that would be the main impact (well, depending on journaling behavior/etc), but if you have more OSDs than that then you could have situations where one file is getting rolled back, and some other file isn't, and so on.
But you’d have a voting majority.
With min_size=2 you're fairly safe from flapping because there will always be two replicas that have the most recent version of every PG, and so you can still tolerate a permanent failure of one of them.
Exactly.
size=2, min=2 doesn't suffer this failure mode, because anytime there is flapping the PG goes inactive and no writes can be made, so when the other OSD comes back up there is nothing to recover. Of course this results in IO blocks and downtime, which is obviously undesirable, but it is likely a more recoverable state than inconsistent writes.
Agreed, the difference between availability and durability. Depends what’s important to you.
Apologies if I've gotten any of that wrong, but my understanding is that it is these sorts of failure modes that cause min_size=1 to be a trap. This isn't the sort of thing that typically happens in a RAID1 config, or at least that admins don't think about.
It’s both.